Magneto-optical disk pick-up device having mode splitter using cut off effect caused by tapered waveguide

ABSTRACT

A mode splitter and the magneto-optical pick-up device including the mode splitter. The mode splitter includes a tapered waveguide portion whose thickness gradually becomes thin so that a light of at least one mode can be cut off. An emission position of a light wave can be varied depending on its mode. In this mode splitter, the emission position of the light wave of each mode can be adjusted by changing a thickness of the tapered waveguide, thereby improving the degree of freedom in designing the device.

This application is a divisional of application Ser. No. 08/213,800filed Mar. 16, 1994 now U.S. Pat. No. 5,509,094.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mode splitter (mode separationdevice) and a magneto-optical disk pick-up device using the modesplitter.

2. Description of the Related Art

As a mode separation device using a flat-plate optical waveguide, a modesplitter of a 90° Bragg type, a directional coupling type or the likehas conventionally been known.

However, a mode splitter of the 90° Bragg type has a disadvantage ofbeing weak against a wavelength shift. Further, the mode splitter of adirectional coupling type requires strict regulation and control of acoupling length and the like during the fabrication thereof, so thatthere are also disadvantages of poor productivity and raising a cost.

Because of the above-mentioned disadvantages, an optical integrateddevice, such as an optical pickup device including a mode splitter isdifficult to be put to a practical use.

In view of such problems, in recent years, a mode splitter using aflat-plate optical waveguide has been improved so as to be applicable tothe optical integrated device. An example of such a mode splitter isshown in FIGS. 12 and 13. This mode splitter has two flat-platewaveguides A and B each having a uniform thickness. The waveguides A andB are combined with each other through a coupler C. The thickness of thecoupler is varied to form a tapered shape in a cross section. In thismode splitter, propagation constants β_(iA), β_(iB), β_(jA), and β_(jB)and an incident angle α_(A) are set in order to satisfy therelationships: β_(iA) >β_(iB), β_(jA) >β_(jB) and α_(A) <arcsin (β_(iB)/β_(iA)), α_(A) >arcsin (β_(jB) /β_(jA)), while at least the light waveof i mode and the light wave of j mode are propagated in the samedirection.

As a result, in the case where, for example, the modes i and j arerespectively represented as a TE₀ mode and a TM₀ mode, the TE₀ mode (aTE wave) is completely reflected on the tapered coupler C and the TM₀mode (a TM wave) is transmitted from the waveguide A to the waveguide B.Accordingly, the TE wave and the TM wave can be separated from eachother (i.e., a deflection separation).

However, there are problems in the above-mentioned mode splitter.

That is, while performing a mode separation in the mode splitter shownin FIGS. 12 and 13, a separated light wave remains to be confined in awaveguide layer. Accordingly, it is required to provide a grating typecoupler or the like in an optical integrated device including such amode splitter for emitting a light wave from a waveguide layer in orderthat the light wave confined in the waveguide layer (i.e., light wave)is carried to a detection system.

Thus, the optical integrated device including the above-mentioned modesplitter has disadvantages that its structure becomes complicated andlarger-sized. Moreover, while emitting the light wave from the waveguidelayer by the grating coupler or the like, the coupling is damaged due tothe wavelength shift or the like.

Furthermore, the light wave is diagonally incident on the taperedcoupler C (see FIG. 12), and the TE wave and the TM wave are separated(deflection separation). A slight shift in an incidental angle makes alarger reflection of the TM wave from the tapered coupler C, namely, theslight shift is a great factor for changing the reflectance of the TMwave. As a result, the extinction ratio (strength of the reflectionlight wave/strength of the transmittance light wave) is deteriorated.

SUMMARY OF THE INVENTION

A mode splitter according to the present invention comprises: awaveguide layer including a uniform waveguide section having asubstantially uniform thickness and a tapered waveguide section which isoptically coupled with the uniform waveguide section, wherein theuniform waveguide section propagates a plurality of light waves ofdifferent modes, and the tapered waveguide section has a thicknessgradually thinned so that the light waves are cut off in the taperedwaveguide section, thereby emitting the light waves from differentpositions of the tapered waveguide section.

In one embodiment of the invention, the mode splitter comprises a firstlayer coupled with the waveguide layer, and a second layer coupled withthe first layer, the second layer including at least one refractingregion for refracting a corresponding one of the light waves emittedfrom the tapered waveguide.

In one embodiment of the invention, the refracting region has a lowerrefractive index than that of the second layer.

In one embodiment of the invention, the mode splitter comprises asubstrate for supporting the first and second layers and the waveguidelayer, the substrate having a higher refractive index than that of therefracting region of the second layer.

In one embodiment of the invention, the tapered waveguide section of thewaveguide layer includes a plurality of tapered portions.

A magneto-optical disk pick-up device according to the present inventioncomprises: a light source for emitting light; a first optical means forfocusing the light on a magneto-optical recording medium; a secondoptical means for guiding the light reflected from the magneto-opticalrecording medium to a servo signal generation means for producing servosignals in response to the light reflected from the magneto-opticalrecording medium; and a mode splitter for receiving the light from thesecond optical means and splitting the light into a plurality of lightwaves of different modes, and guiding the plurality of light waves to amagneto-optical information reproduction means, the mode splittercomprising: a waveguide layer including an uniform waveguide sectionhaving a substantially uniform thickness and a tapered waveguide sectionwhich is optically coupled with the uniform waveguide section, whereinthe uniform waveguide section propagates the light from the secondoptical system, and the tapered waveguide section has a thicknessgradually thinned so that the light waves are cut off in the taperedwaveguide section, thereby emitting the light waves from differentpositions of the tapered waveguide section.

In one embodiment of the invention, the magneto-optical pick-up devicecomprises: a first layer coupled with the waveguide layer; and a secondlayer coupled with the first layer, the second layer including at leastone refracting region for refracting corresponding one of the lightwaves emitted from the tapered waveguide.

In one embodiment of the invention, the refracting region has a lowerrefractive index than that of the second layer.

In one embodiment of the invention, the magneto-optical pick-up devicecomprises a substrate for supporting the first and second layers and thewaveguide layer, the substrate having a higher refractive index thanthat of the refracting region of the second layer.

In one embodiment of the invention, the tapered waveguide section of thewaveguide layer includes a plurality of tapered portions.

In a flat-shaped optical waveguide in which light waves of an i mode andj mode each having a different propagation constant β (β_(i) ≠β_(j)) canbe propagated, a waveguide section having a uniform thickness is coupledto a tapered waveguide portion whose thickness gradually becomes thin ina tapered shape so that a light wave of at least one mode can be cutoff. As a result, an emission position of the light wave can be varieddepending on the mode. That is, mode separation can be performed. Insuch a mode splitter, the emission position of a light wave of each modecan be varied by changing the thickness of the tapered waveguideportion.

Moreover, a light wave can be emitted by using emission from the taperedwaveguide portion in the waveguide, so that a means for emitting thelight wave such as a grating coupler is not required, different from theconventional mode splitter including the tapered coupler C. Accordingly,the structure of the device can be made compact and the emission of alight wave by the coupler is not influenced due to any wavelength shiftor the like.

Further, in the case of using the mode splitter of the present inventionas a polarization separation device, it is not required that a lightwave is diagonally incident on the tapered waveguide, which is differentfrom the conventional mode splitter. Thus, deterioration of theextinction ratio caused by a slight change in the incident angle canthoroughly be prevented.

According to the magneto-optical disk pick-up device of the presentinvention, the servo signal generation system such as a photo diode andthe magneto-optical information reproduction system can be integratedlymounted with the mode splitter. Accordingly, the magneto-optical diskpick-up device can be made compact.

Thus, the invention described herein makes possible advantages of (1)providing a mode splitter of an optical waveguide type in which lightwaves of at least two modes each having a different propagationconstant, propagating in the optical waveguide in the same direction canbe separated without being influenced by a wavelength shift and lightwave confined in a waveguide layer can be emitted from the waveguidelayer without using a coupler and (2) providing a magneto-optical diskpick-up device in which the structure of the device can be made compactby providing the mode splitter therein.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first example of a mode splitteraccording to the present invention.

FIG. 2 is a cross-sectional view showing the way of emission of a TEwave and a TM wave.

FIG. 3 is graph showing distribution of the strength of a y component ofelectric fields of the TE wave depending on x position.

FIG. 4 is a cross-sectional view of an intermediate layer in the modesplitter of the present invention.

FIG. 5 is a partial cross-sectional view of a refractive region of themode splitter according to the present invention.

FIG. 6 is a cross-sectional view of a second example of a mode splitteraccording to the present invention.

FIG. 7 is a cross-sectional view of a third example of a mode splitteraccording to the present invention.

FIG. 8 is a schematic side view of a magneto-optical disk pick-up deviceaccording to the present invention.

FIGS. 9A, 9B and 9C are views showing the relationship between amagnetization direction of a magneto-optical disk and a rotation of apolarization plane.

FIG. 10 is a perspective view showing an enlargement of a servo signalgeneration system of the magneto-optical disk pick-up device as shown inFIG. 8.

FIG. 11 is a schematic side view of another magneto-optical disk pick-updevice according to the present invention.

FIG. 12 is a plan view showing a conventional mode splitter.

FIG. 13 is a side view of the mode splitter of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way of examples,with reference to the accompanying drawings.

EXAMPLE 1

FIGS. 1 through 3 show a first example of a mode splitter-according tothe present invention. This example is a mode splitter which is appliedto a polarized light separation device for separating a light wave of aTE₀ mode and a light wave of a TM₀ mode. Here, the light wave of the TE₀mode (propagation constant: β_(TE)) and a light wave of a TM₀ mode(propagation constant: β_(TM)) are propagated through a waveguide in themode splitter. The structure of mode splitter will be now described indetail.

The mode splitter has a flat-plate substrate 4, on which a secondintermediate layer 3, a first intermediate layer 2 and a waveguide layer1 are formed in this order. The substrate 4 is made of a material with arefractive index of n₄. The second intermediate layer 3 of a flat plateshape is made of a material having a refractive index of n₃ and auniform thickness of t₂. The second intermediate layer 3 includes tworefractive regions 5 of a parallelogram shape formed therein with anappropriate distance therebetween. Each refractive region 5 is made of amaterial having a refractive index of n₅ and a width of b, inclined atθ_(t) with respect to the second intermediate layer 3. The firstintermediate layer 2 of a flat plate shape is made of a material havinga refractive index of n₂ and an uniform thickness of t₁. The waveguidelayer 1 made of a material having a refractive index of n₁ includes atapered portions 1A having a gradient angle φ.

The tapered portion 1A is positioned above both of the two refractiveregions 5 and coupled to a uniform waveguide portion 1B having anuniform thickness of t_(W). A light wave which is incident on thewaveguide layer 1 propagates through the uniform waveguide portion 1Binto the tapered portion 1A, where the light wave is split intodifferent polarized light waves in accordance with the modes thereof. Inthis example, the thickness of t_(W) of the uniform waveguide portion 1Bis set so that only the light wave of the TE₀ mode and that of the TM₀mode can be propagated.

The second intermediate layer 3 is provided so as to refract a lightwave which is incident thereon toward a desired direction. For the sakeof preferable refraction, the refractive indexes n₅, n₄ and n₃ have thefollowing relationship represented by the Relationship (1).

    n.sub.5 <n.sub.3, n.sub.5 <n.sub.4                         (1)

The substrate 4 having the refractive index n₄ serves to increase therefractive effect of the refractive regions 5.

The principle of the mode separation in the mode splitter will be nowdescribed with reference to FIG. 2. In general, as the thickness of awaveguide layer where light wave is mainly propagated is decreased, thedegree of the light wave confinement within the waveguide layer isdecreased. If the thickness of the waveguide is decreased to be lessthan a certain thickness, the light wave can not propagate at all in thewaveguide and be emitted therefrom. This phenomenon is called as acut-off phenomenon. In FIG. 2, three-dimensional orthogonal coordinateaxes (x, y, z) are shown, in which the x direction indicates thethickness direction of the waveguide layer 1; the y direction indicatesthe width direction of the waveguide; and the z direction indicates thepropagation direction of light waves in the waveguide layer 1.

In the case where the light waves of the TE₀ mode and the TM₀ modepropagate in the waveguide, and the propagation constant of the TE₀ mode(β_(TE)) and that of the TM₀ mode (β_(TM)) satisfy a relationship ofβ_(TE) >β_(TM), a layer thickness d_(TE) and d_(TM) of the taperedportion 1A at which the light wave of each mode is cut off are different(d_(TE) <d_(TM)). In this mode splitter, the thickness of the waveguidelayer 1 gradually becomes thin along the propagation direction of thelight wave in the waveguide layer 1. As a result, when the thickness ofthe waveguide layer 1 is in the vicinity of d_(TM), the TM wave is firstemitted to the first intermediate layer 2. After that, only the TE wavecan propagate in the waveguide layer 1. The thickness of the taperedportion 1A is further thinned and the TE wave is also emitted to thefirst intermediate layer 2 when the thickness is in the vicinity ofd_(TE).

As is apparent from the above, in the above-mentioned mode splitter, theTM wave and the TE wave are emitted at different positions of thewaveguide layer 1, so that the mode separation (polarized lightseparation) can be realized. A distance f between the emission positionsof the both waves is represented by using the gradient angle φ of thetapered portion 1A (FIG. 2) as in the following.

    f=(d.sub.TM -d.sub.TE) tan φ                           (2)

In the mode splitter, the emission position of each mode wave can becontrolled as desired by appropriately adjusting the gradient angle φ ofthe tapered portion 1A.

Next, a preferable condition for the first intermediate layer 2 and afunction of the second intermediate layer 3 will be explained byillustrating the emission of the TM wave.

Firstly, a preferable condition for the thickness t₁ of the firstintermediate layer 2 will be explained with reference to FIG. 3. FIG. 3shows a distribution of the y directional component Ey in electricfields of the TE wave depending on the x position along the thicknessdirection of the waveguide.

As is understood from FIG. 3, the E_(y) directional component leakstoward the substrate 4 by a distance d₁. In order to avoid any influencecaused by the leakage (i.e., in order to prevent light wave propagatedthrough the waveguide layer 1 from being influenced by the refractiveregions 5 provided in the second intermediate layer 3), the thickness t₁of the first intermediate layer 2 should be set at t₁ >d₁.

The provision of the second intermediate layer 3 is useful for thefollowing reasons. An emission angle φ_(r) of the TM wave emitted fromthe waveguide layer 1 (i.e., an angle formed by a bottom surface of thewaveguide layer 1 and the emission direction of the TM wave) is quitesmall as shown in FIG. 4. Accordingly, a degree of freedom in adirection in which light wave is emitted from the mode splitter issmall, resulting in poor efficiency in emission of the light wave fromthe device. However, by providing the second intermediate layer 3 havingthe refractive regions 5, the TM wave emitted from the firstintermediate layer 2 is refracted on a boundary between the secondintermediate layer 3 and the refractive region 5 and the refracted lightwave is further refracted on a boundary between the second intermediatelayer 3 and the substrate 4 (double refraction), whereby the light wavecan be emitted from the mode splitter with a preferable emission angleθ₀. In this case, the degree of freedom in the emission direction oflight wave can be improved, resulting in the emission of light wavesfrom the mode splitter with satisfactory efficiency.

Preferable conditions for parameters regarding a configuration of thesecond intermediate layer 3 will be explained with reference to FIGS. 4and 5. The parameters include the thickness t₂ of the secondintermediate layer 3, a gradient angle θ_(t) and a width of b of therefractive region 5, and a distance 1 between the refractive region 5and an emission ending position z_(end).

The light wave emitted from the waveguide layer 1 should propagatetoward a negative x direction (the substrate 4) by at least thethickness t₁ of the first intermediate layer 2. Accordingly, thedistance 1 should be set to satisfy the relationship of 1>t₁ / sinθ_(r).

Moreover, the gradient angle θ_(t), a desired emission angle θ₀ from therefractive region 5 to the substrate 4 and a refractive angle θ_(y) ofthe refractive region 5 (see FIG. 5) are set so as to satisfy thefollowing Formulas (3) and (4):

    θ.sub.0 =arcsin {n.sub.5 cos (θ.sub.t -θ.sub.y)/n.sub.4 }(3)

    θ.sub.y =arcsin {n.sub.3 cos (θ.sub.t -θ.sub.r)/n.sub.5 }(4)

Furthermore, a value for each thickness t₂ and the width b should be setso that emitted light waves can sufficiently be propagated through therefractive region 5. In the case where a desired distance from an edgeof the refractive region 5 to an emission position of light wave isz_(p) (z_(p) >r_(i)) as shown in FIG. 5, the thickness t₂ and the widthb should be set to satisfy the following relationships represented bythe Formulas (5) and (6):

    t.sub.2 >x.sub.p, x.sub.p =tan (θ.sub.t -θ.sub.y)·tan θ.sub.t ·z.sub.p /{tan θ.sub.t -tan (θ.sub.t -θ.sub.y)}                                          (5)

    b>z.sub.p                                                  (6)

wherein r_(i) is a beam diameter of light wave incident on therefractive region 5.

The refractive region 5 with the TE wave has the same parameters asthose of the refractive region 5 with the TM wave.

As is understood from FIG. 4, an emission angle θ_(R) from the emissionstarting position z_(ST) of the waveguide layer 1 is larger than theemission angle θ_(r) at the emission ending position z_(end), so thatlight wave is inevitably converged in the z direction. Accordingly, ifthe waveguide layer 1 has a light-converging effect in the y direction,the emitted light wave can be converged.

EXAMPLE 2

FIG. 6 shows a second example of a mode splitter according to thepresent invention. The mode splitter includes a waveguide layer 11having two tapered portions 11A and 11B. Other components of the deviceare the same as those in Example 1. A detailed explanation for eachcorresponding element is omitted.

The principle of the mode separation will be now explained. The firsttapered portion 11A provided on the left side of the waveguide layer 11has a thickness which gradually becomes thin from t_(w) to h (d_(TE)<h<d_(TM)) in the right side, following the direction in which lightwave is propagated in accordance with a gradient angle φ". The secondtapered portion 11B is formed with a gradient angle φ'. The waveguidelayer 11 has a flat waveguide layer portion 12 with a length of L and athickness of h interposed between the first tapered portion 11A and thesecond tapered portion 11B.

In the case where the TE wave and the TM wave are propagated through thewaveguide layer 11, the thickness h is set so that the light wave of theTM₀ mode is cut off and only the light wave of the TE₀ mode can bepropagated.

As a result, the TM wave is emitted from the vicinity of the cut-offposition to the substrate 4 through a path 6. Accordingly, only the TEwave is propagated into the flat waveguide layer portion 12 coupled tothe tapered portion 11A. Similarly, the TE wave is propagated throughthe tapered portion 11B and then emitted from the vicinity of thecut-off position to the substrate 4 through a path 7.

In this example, there is an advantage that by appropriately adjustingthe length L, a distance between the emission position of the TE waveand that of the TM wave can be set with independence of the gradientangle φ" and the gradient angle φ'. Thus, the degree of freedom indesigning the device, thereby improving the formation thereof.

EXAMPLE 3

FIG. 7 shows a third example of a mode splitter according to the presentinvention. The mode splitter has the same second intermediate layer 13as that of the mode splitter in Example 2 except that an aperture 8(refractive index n_(a) =1.0) for emitting the TM wave is providedtherein. Due to the aperture 8, the refractive index for the light wavecan be made larger,-compared with the refractive index of the refractiveregion 5. As a result, the emission angle θ₀ from the device can be madelarger.

Furthermore, edges of the first intermediate layer 2 and the secondintermediate layer 13 are diagonally cut off with an angle θ_(t) asshown in FIG. 7, thereby forming the same aperture on the right side ofthe edges. Thus, the same effects as the above can be obtained.

EXAMPLE 4

FIG. 8 shows a fourth example of the present invention, in which themode splitter of the present invention is applied to a magneto-opticalpick-up device.

First, the structure of the magneto-optical pick-up device will bedescribed. The magneto-optical pick-up device has a substrate 114 onwhich an intermediate layer 113 and a waveguide layer 111 are formed.The waveguide layer 111 includes a flat section and a tapered section. Aglass substrate 109 is mounted on the flat section of the waveguidelayer 111. A hologram 103 is formed on an almost middle portion of theupper surface of the glass substrate 109, and a tracking beam generationgrating 105 is formed on a back surface of the glass substrate 109 belowthe hologram 103.

A mode splitter 112 which is provided on the right side of the glasssubstrate 109 includes the substrate 114, the intermediate layer 113 andthe waveguide layer 111. The waveguide layer 111 in the mode splitter112 has two tapered portions 111A and 111B interposing a flat waveguidelayer 111C portion therebetween.

A semiconductor laser 106 as a light source is provided below thetracking beam generation grating 105 and outside the substrate 114. Adivided hologram 104 including two holograms each having a differentgrating period is provided on a left of the back surface of thesubstrate 114. A five-divided photo diode (PD) 108 for detecting a minus1st-order diffraction light wave from the hologram 103 so as to producea servo signal is provided diagonally below the hologram 104.

A light coupler 107 is provided on the waveguide layer 111 at the rightside of the tracking beam generation grating 105. The light coupler 107leads a plus 1st-order diffraction light waive from the hologram 103 tothe mode splitter 112. A PD 116 and a PD 117 are provided on a backsurface of the substrate 114. The PD 116 detects a P wave (TM wave) ofthe plus 1st-order diffraction light wave from the left refractiveregion 115. The PD 117 detects an S wave (TE wave) of the plus 1st-orderdiffraction light wave from the right refractive region 115.

Further, a collimator lens 102, an objective lens 101 and amagneto-optical disk 118 are provided above the hologram 103 in thisorder.

The operation of the magneto-optical disk pick-up device will be nowdescribed. A light wave emitted from the semiconductor laser 106 isfirst divided into a main beam and a sub beam by the tracking beamgeneration grating 105. The main beam is incident on the objective lens101 through the hologram 103 and the collimator lens 102, thereby beingconverged in an information track of the magneto-optical disk 118. Thepolarization plane of the light wave rotates by the Ker effect at themagneto-optical disk 118, and the light wave is reflected by themagneto-optical disk 118 so as to be led to the hologram 103 through theobjective lens 101 and the collimator lens 102. The reflected light waveof the main beam is diffracted by the hologram 103, thereby generatingthe plus 1st-order diffraction light wave and the minus 1st-orderdiffraction light wave. Moreover, the sub beam is diffracted in the sameway as the main beam.

Next, the plus 1st-order diffraction light wave is led to the modesplitter 112 through the light coupler 107, whereby a light wave of theTE₀ mode and a light wave of the TM₀ mode are excited in the waveguidelayer 111 by the S wave and the P wave, respectively. As a result, theTM wave and the TE wave are detected by the PD 116 and the PD 117,respectively, based on the mode separation function of the mode splitter112. In response to the intensity of each light wave, the PD 116 and thePD 117 photoelectrically produce detection signals and send them to asignal processing system (not shown). Based on the detection signalsoutput from the PD 116 and PD 117, information recorded on themagneto-optical disk 118 (magneto-optical signals) is reproduced.

The reproduction mechanism of this will be explained in more detail withreference to FIG. 9. In the case where a polarization plane of a laserlight wave is previously rotated at 45° as shown in FIG. 9A, thedirection of the polarization plane is varied depending on themagnetization direction of a recording layer of the magneto-optical disk118 as shown in FIGS. 9B and 9C for comparison. As a result, thestrength of the S wave component and that of the P wave component aredifferent between FIG. 9B and FIG. 9C. For example, in the case of FIG.9B, the detected level of the S wave component is "L" (=LOW) level andthe detected level of the P wave component is "H" (HIGH) level. On thecontrary, in the case of FIG. 9C, the detected level of the S wavecomponent is "H" level and the detected level of the P wave component is"L" level. Accordingly, the detection signals output from the PD 116 andPD 117 can reproduce the magneto-optical signals.

On the other hand, the minus 1st-order diffraction light wave generatedby the hologram 103 is led to the five-divided PD 108 together with themain beam and the sub beam through the divided hologram 104, therebyproducing the servo signals. The principle will be described below withreference to FIG. 10.

The divided hologram 104 is divided by the division line 104a into tworegions each having a different grating period. The five-divided PD 108is divided into five light detecting portions D₁, D₂, D₃, D₄ and D₅ withrespect to the division line 104a. Accordingly, the reflected light waveof the main beam which is incident on one region of the divided hologram104 is focused on the division line between the light detecting portionsD₂ and D₃. Further, another reflected light wave of the main beam whichis incident on the other region of the divided hologram 104 is focusedon the light detecting portion D4. Furthermore, each reflected lightwave of the sub beam is focused on the light detecting portions D₁ andD₅.

Now, an output (photoelectrically converted electrical signal) from eachsegment (light detecting portion) of the five-divided PD 108 is S₁, S₂,S₃, S₄, and S₅. A focusing error signal (FES) indicates whether lightconverged on the magneto-optical disk 118 by the objective lens 101 isproperly focused thereon or not. The FES is represented by the followingFormula (7) based on the Foucault method.

    FES=S.sub.2 -S.sub.3                                       (7)

Further, a radial error signal (RES) for indicating whether a trackingis correctly carried out or not is represented as the following Formula(8) based on the three-beams method.

    RES=S.sub.1 -S.sub.5                                       (8)

A detailed explanation of a generation method of the servo signal isdisclosed in, for example, Japanese Laid-Open Patent Publication No.1-151022.

In this magneto-optical disk pick-up device, the servo signal generationsystem including the divided hologram 104, the five-divided PD 108 andthe like are combined with a magneto-optical signal generation systemincluding the light coupler 107, the PD 116, the PD 117 and the like.Accordingly, the magneto-optical disk pick-up device of this example canbe more miniaturized compared to a conventional magneto-optical diskpick-up device.

EXAMPLE 5

FIG. 11 shows a fifth example of the present invention, in which themode splitter is applied to the magneto-optical disk pick-up device inanother manner.

In this example, a hologram 123 corresponding to the hologram 103 inExample 4 is made to be blazed (i.e., a grating shape thereof is changedinto a sawtooth shape), thereby suppressing generation of a minus1st-order diffraction light wave. Thus, a plus 1st-order diffractionlight wave can be more effectively led to the light coupler 107 comparedwith Example 4.

Part of the 1st-order diffraction light wave is not coupled with theoptical waveguide of the mode splitter 112, but passes through the lightcoupler 107. This part of the 1st-order diffraction light wave is led tothe five-divided PD 128 through a divided hologram 124, therebyproducing the servo signal. The magneto-optical disk pick-up device ofthis example has a divided hologram provided on a back face of thesubstrate 114 diagonally below the light coupler 107 and a five-dividedPD 128 provided outside the substrate 114 diagonally below the dividedhologram 124.

According to the magneto-optical disk pick-up device of Example 5, thereare advantages of reducing the size of a magneto-optical disk pick-updevice as in Example 4 and realizing a magneto-optical disk pick-updevice in which utilization efficiency of the light wave can be furtherimproved.

As described above, the mode splitter according to the present inventionincludes a tapered waveguide portion whose thickness gradually becomesthin so that a light wave of at least one mode can be cut off. As aresult, an emission position of a light wave can be varied depending onits mode. Accordingly, in this mode splitter, the emission position ofthe light wave of each mode can be set by changing the thickness of thetapered waveguide, thereby improving the degree of freedom of thestructure of the device.

Moreover, a light wave of a desired mode can be obtained by using aradiation from the tapered waveguide portion, so that a coupler foremitting the light wave is not required. Accordingly, a bad influencecaused by a wavelength shift can be removed and the mode splitter can bemade compact.

In the case of using the mode splitter as a polarization separationdevice, it is not required for a light wave to be diagonally incident onthe tapered waveguide portion. As a result, the deterioration of theextinction ratio is avoided.

In another aspect of the invention, a plurality of refractive regionseach having a different refractive index are provided in a secondintermediate layer on the side where a light wave is emitted from awaveguide layer. As a result, the degree of freedom in an emissiondirection of light wave can be improved, whereby the emission of lightwave can be facilitated. Accordingly, the mode splitter can be expectedto be applied to various optical integrated devices such as amagneto-optical disk pick-up device.

In a mode splitter which includes a plurality of tapered waveguideportions, the emission position of a light wave of each mode can beregulated independently of a gradient of the tapered waveguide portion.As a result, the degree of freedom in the structure of the device can befurther improved.

An magneto-optical disk pick-up device according to the inventionincludes the mode splitter of the invention so that the servo signalgeneration system can easily be combined with the magnetic signalgeneration system, thereby reducing the size of the device and reducingcost.

Various other modification will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A magneto-optical disk pick-up device,comprising:a light source for emitting light; a first optical means forfocusing the light on a magneto-optical recording medium; a secondoptical means for guiding the light reflected from the magneto-opticalrecording medium to a servo signal generation means for producing servosignals in response to the light reflected from the magneto-opticalrecording medium; and a mode splitter for receiving the light from thesecond optical means and splitting the light into a plurality of lightwaves of different modes, and guiding the plurality of light waves to amagneto-optical information reproduction means,the mode splittercomprising: a waveguide layer including an uniform waveguide sectionhaving a substantially uniform thickness and at least one taperedwaveguide section which is optically coupled with the uniform waveguidesection,wherein the uniform waveguide section propagates the light fromthe second optical system, and the tapered waveguide section has athickness gradually thinned so that each of the light waves of differentmodes is respectively cut off at different positions in the taperedwaveguide section, thereby emitting each of the light waves from therespective different positions of the tapered waveguide section.
 2. Amagneto-optical disk pick-up device according to claim 1 comprising:afirst layer formed beneath the waveguide layer; and a second layerformed beneath the first layer, the second layer including at least onerefracting region for refracting a corresponding one of the light wavesemitted from the tapered waveguide.
 3. A magneto-optical disk pick-updevice according to claim 2, wherein the refracting region has a lowerrefractive index than that of the second layer.
 4. A magneto-opticaldisk pick-up device according to claim 3 comprising:a substrate forsupporting the first and second layers and the waveguide layer, thesubstrate having a higher refractive index than that of the refractingregion of the second layer.
 5. A magneto-optical disk pick-up deviceaccording to claim 1, wherein the plurality of light waves of differentmodes includes a TM mode wave and a TE mode wave.
 6. A magneto-opticaldisk pick-up device according to claim 1, wherein the waveguide layerincludes:a first uniform waveguide section having a first thickness; asecond uniform waveguide section having a second thickness; a firsttapered waveguide section interposed between the first and seconduniform waveguide sections and coupled therewith, a thickness of thefirst tapered waveguide section gradually becoming thinner from thefirst thickness to the second thickness; and a second tapered waveguidesection coupled with the second uniform waveguide section, the first andsecond tapered waveguide sections sandwiching the second uniformwaveguide section, and a thickness of the second tapered waveguidesection gradually becoming thinner from the second thickness.
 7. Amagneto-optical disk pick-up device according to claim 6, wherein agradient angle of the first tapered waveguide section is different fromthat of the second tapered waveguide section.
 8. A magneto-optical diskpick-up device according to claim 4, wherein the refracting region has ashape in which one side thereof is inclined with respect to the secondlayer at an angle of θ_(t), the angle of θ_(t) being set so as tosatisfy the following equations:

    θ.sub.o =arcsin {n.sub.5 ·cos (θ.sub.t -θ.sub.r)/n.sub.4)

    θ.sub.y =arcsin {n.sub.3 ·cos (θ.sub.t -θ.sub.r)/n.sub.5)

where n₃ is the refractive index of the second layer, n₄ is therefractive index of the substrate, n₅ is the refractive index of therefracting region, θ_(y) is a refractive angle of the refracting region,θ_(o) is an emission angle from the refracting region and θ_(r) is anemission angle from the tapered waveguide section of the waveguidelayer.
 9. A magneto-optical disk pick-up device according to claim 4,further comprising a plurality of photo detectors for detecting thelight waves emitted from the intermediate layer, the photo detectorsbeing provided in such a manner that the photo detectors oppose to theintermediate layer with the substrate interposed therebetween,whereinthe intermediate layer leads the light waves emitted from the taperedwaveguide sections toward the photo detectors.
 10. A magneto-opticaldisk pick-up device according to claim 1, wherein the tapered waveguidesection of the waveguide layer includes a plurality of tapered portions.11. A magneto-optical disk pick-up device according to claim 1, whereineach of the light waves from the respective different positions of thetapered waveguide section is emitted in a direction transverse to theboundary between the waveguide layer and the first layer formed beneaththe waveguide layer.
 12. A magneto-optical disk pick-up device accordingto claim 2, wherein the refracting region is made of material which isdifferent from that of the second layer.
 13. A magneto-optical diskpick-up device, comprising:a light source for emitting light; a firstoptical means for focusing the light on a magneto-optical recordingmedium; a second optical means for guiding the light reflected from themagneto-optical recording medium to a servo signal generation means forproducing servo signals in response to the light reflected from themagneto-optical recording medium; a mode splitter for receiving thelight from the second optical means and splitting the light into aplurality of light waves of different modes, and directing the pluralityof light waves toward different positions,the mode splitter comprising:a waveguide layer including an uniform waveguide section having asubstantially uniform thickness and a tapered waveguide section which isoptically coupled with the uniform waveguide section,wherein the uniformwaveguide section propagates the light from the second optical system,and the tapered waveguide section has a thickness gradually thinned sothat the light waves are cut off in the tapered waveguide section,thereby emitting the light waves from different positions of the taperedwaveguide section; and a magneto-optical information reproduction means,comprising:a plurality of photo detectors for detecting each of thelight waves of the different modes which are emitted from the taperedwaveguide section, the photo detectors being located under the waveguidelayer.